1. Field of the Invention
The present invention relates generally to video signal processing, and more particularly, to detection, correction and display of video signals.
2. State of the Art
During video signal processing, numerous data formats are used to represent image information associated with each pixel of a video field so that an original image can be faithfully reproduced. For example, one common color format represents a color using red, green, and blue color components. With this color format, the color of each pixel is represented by quantities of red (R), green (G) and blue (B) color components detected in the original image (referred to hereafter as an R,G,B format). Another format used to represent a particular color is by the amounts of hue (.crclbar.), saturation (S), and luminance (Y) included therein (referred to herein as the Y,S, .crclbar. format).
Table 1 lists these two color formats, along with three additional color formats:
TABLE 1 ______________________________________ Rred Yluminance Yluminance Yluminance Yluminance ______________________________________ Ggreen Ssaturation Cchrominance Uin phase Crin phase chroma chroma Bblue .THETA.hue .THETA.hue Vquad Cbquad phase phase chroma chroma ______________________________________
A series of mathematical transformations exists between these various color formats. However, because each of these color formats is implemented differently in the real world, conflicts exist in moving between them. These conflicts derive from the fact that different color formats possess different characteristics, such as analog versus digital characteristics and component signal versus composite signal characteristics.
For example, the R,G,B color format is an analog format because the video signals are analog in nature and vary, for example, between 0 and +0.7 volts. Further, the R,G,B format is a component format because the entire video signal is transmitted in 3 component parts using three separate R,G,B signal paths. A common use of this transmission format is in computer graphics equipment and computer displays. This can be contrasted with a composite video signal, where the video signal is contained and transmitted in its entirety on a single signal path.
A Y,U,V format is an analog component format which is very similar to the R,G,B, format. The Y,U,V format includes a luminance (Y) component, an in phase chroma component (U) and a quadrature phase component (V). This format separates luminance (or brightness) information from chrominance (or color) information, wherein Y represents luminance, and U and V together represent chrominance. The Y,U,V format is sometimes preferred over the R,G,B format because a black and white monitor can be driven with just the Y signal. A transformation between the R,G,B and Y,U,V format is as follows: EQU Y=0.299 * R+0.587 * G+0.144 * B EQU U=-0.148 * R-0.289 * G+0.437 * B 0.ltoreq.R, G, B,.ltoreq.1 EQU V=0.615 * R-0.515 * G-0.100 * B
A Y,Cr,Cb format is a digital component format specified by the international standard set forth in CCIR 601/656 and frequently referred to as the 4:2:2 format. Generally speaking, the Y,Cr,Cb components are digital versions of the Y,U,V analog components.
The "4:2:2" designation refers to the use of a sampling rate for U and V components which is one half the sample rate for the Y component. For example, the Y component is sampled at 13.5 Mhz and the U and V components are sampled at 6.75 Mhz each. The precision of the digitization is typically 10 bits.
The 4:2:2 format is considered a high quality format and is therefore preferred for use in high end post production studios. Video signals received by a post production studio in another color format are typically transformed or converted from other color formats to the 4:2:2 format before editing begins. For example, analog components of the Y,U,V format can be converted to components in the 4:2:2 format using the following mathematical transformation: EQU Y(4:2:2)=Y * 876+64 decimal EQU Cr=((V/1.2296)+0.5) 896+64 decimal EQU Cb=((U/0.8736)+0.5) 896+64 decimal
In contrast to component formats, a composite format encodes an entire video signal onto one signal path. An analog composite format is used primarily to distribute a video signal from an originating facility to a viewer where it is decoded and displayed on a television receiver. There are two primary analog composite formats: NTSC, used mainly in North America and Japan, and PAL, versions of which are used throughout Asia and Europe. The composite NTSC format can be derived from the analog component Y,U,V format using the following mathematical transformation: EQU Amplitude=[(Y+C) * .0925+0.075], 0.714 volts EQU C=V * sin(wt+0.576)+U * cos(wt+0.576) EQU .omega.=3.579545 MHz
A similar derivation exists for transforming the composite PAL format from the analog component Y,U,V format.
A typical use of various color formats in a post production studio is illustrated in FIG. 1. As mentioned previously, real world implementations impose restrictions on final values derived when transforming one color format into another color format. These restrictions can render a color which is legal in one color format, illegal in another color format.
As referenced herein, a "legal color" is a color which can be accurately reproduced by conventional video equipment in a given color format. An "illegal color" is a color which is outside the color space of conventional video equipment and which cannot be faithfully reproduced by the video equipment.
For example, where the Y,Cr,Cb components of the 4:2:2 format are each represented with a 10 bit word, each of these words must remain between a digital value of 0 and a digital value of 1024 decimal. In a typical analog R,G,B format, the individual R,G,B component signals must remain between 0 and 700 mV. The analog composite NTSC signal must typically remain between -142 mV and 785 mV (the limits for the PAL analog composite signal are slightly different). Transforming a legal 4:2:2 signal to one of these analog formats does not guarantee a legal analog signal. However, transformations between these various formats often can not be avoided.
For example, the output format of a post production studio must be analog composite to be compatible with typical end user (e.g., viewer) equipment. Because some post production studios prefer using the higher quality 4:2:2 digital component format for editing, transformation from the 4:2:2 format must be performed once all editing has been completed.
If an illegal analog composite signal exceeds predetermined limits (e.g., color space limits of conventional video equipment), the resultant video waveform will be distorted. For example, voltages greater than 785 mV (for NTSC) frequently stress the dynamic range of video tape recorders and clip the video signal being reproduced. Such clipping renders the resulting color of the video signal unpredictable.
Often the result of distortion due to illegal colors can be much more serious, causing entire portions of the picture to be significantly different from what was intended. For example, a pixel that is 100% amplitude yellow corresponds to legal values of 100% Red and 100% Green in the R,G,B component format. When this pixel is converted to the 4:2:2 format and then to the NTSC composite analog format, the peak voltage levels are 935 mV and 343 mV. 935 mV is significantly above a legal NTSC signal amplitude of 785 mV and cannot be represented in the NTSC format. In other words, 100% yellow is a legal R,G,B color but an illegal NTSC color.
If the video image is edited in an analog composite format, the editor can simply ensure that no signals above (or below) a certain limit are created. However, because editing in a 4:2:2 post production studio is done in a digital component format, the editor does not know what the peak analog composite levels will be after conversion. Further, each individual pixel in the video field has a different peak level and it is impossible for the editor to individually track every one. Thus, in a 4:2:2 post production studio, it is likely that colors the editor creates or acquires from other color formats cannot be represented in the analog composite format.
A first conventional approach used to address the foregoing problem takes the video signal in the 4:2:2 format and, at various monitoring points in the post production studio, converts it to an analog composite format. The analog composite signal can then be viewed on a traditional waveform monitor and the peak excursions measured to identify illegal colors.
There are at least two disadvantages to this first conventional approach. First, the waveform monitor displays all pixels of the video display in real time as they occur. For NTSC, there are approximately 14 million pixels every second. At this rate, it is impossible for the editor viewing the video signal waveform to detect every illegal color. Secondly, this conventional approach can only notify the editor that an illegal color has been detected.
A second conventional approach is referenced in a document entitled "The Chroma Predictor", from Encore Video Industries in Hollywood, Calif. This document describes receiving a video signal in the 4:2:2 format. Pixels of the video signal which are determined to be illegal are corrected to the nearest legal color of the 4:2:2 format in real time and merged with the legal 4:2:2 signal at an output. The result is a signal in the 4:2:2 format that is guaranteed to be legal when a final transformation to an analog composite format is made.
A key feature of this second conventional approach is that chrominance is reduced to render an illegal color legal, while hue and luminance are maintained constant. For example, once a color associated with a given pixel is determined to be illegal, the two chrominance values are adjusted as follows: EQU Cr'=X * (Cr-512)+512 decimal EQU Cb'=X * (Cb-512)+512 decimal
where Cr', Cb' are "legalized" versions of Cr and Cb; Y remains unchanged to preserve constant luminance. Further, the ratio between Cr and Cb remains unchanged to preserve constant hue. For an NTSC composite input video signal (V.sub.in), X is determined as follows:
______________________________________ when Vin(high) &gt; HARDLIMIT(high): for NTSC X = [(HARDLIMIT(high)/7.14 - 7.5)/92.5 - Py]/C when Vin(low) &lt; HARDLIMIT(low): for NTSC X = [(7.5 - HARDLIMIT(low)/7.14)/92.5 + Py]/C where: Py = (Y (4:2:2.) - 64)/876 Pr = (Cr - 64)/896-0.5 Pb = (Cb - 64)/896 - 0.5 U = 0.874 * Pb V = 1.23 * Pr C = (U 2 + V 2] 0.5 HARDLIMITS are in millivolts ______________________________________
The disadvantage of this second approach is that it does not maintain the contrast of the original image in all areas of the image. For example, if an image of the sun contained many different levels of brightness, all of which are determined to be illegal, the entire image would be clipped to the HARDLIMIT value in the foregoing equation. This would distort the image by eliminating some or all of the original contrast.
In addition to the distortions which result from transforming illegal colors, conventional digital video processing systems are also susceptible to other drawbacks. For example, conventional systems used for processing a 4:2:2 digital video signal are unable to accurately detect, identify and log digital data errors relative to frames of the video signal in which they exist.
Conventional video processing systems merely tell an editor where errors occurred in a very general sense; i.e., by identifying an amount of time since the last error was detected. These systems do not describe exactly what video frames contain errors. Further, they do not tell the editor if any other errors occurred before the last error. Thus, errors that occur over a period of time are not individually noted and logged.
Another problem with video signal processing in a post production studio is the routing of the video signal from place to place using digital formats. This routing of digital signals hinders any useful display of the data for analytical or qualatative purposes.
Accordingly, there is a need for more effective processing, analysis and display of digital video data.